Add configurable block replayer (#2863)

## Issue Addressed

Successor to #2431

## Proposed Changes

* Add a `BlockReplayer` struct to abstract over the intricacies of calling `per_slot_processing` and `per_block_processing` while avoiding unnecessary tree hashing.
* Add a variant of the forwards state root iterator that does not require an `end_state`.
* Use the `BlockReplayer` when reconstructing states in the database. Use the efficient forwards iterator for frozen states.
* Refactor the iterators to remove `Arc<HotColdDB>` (this seems to be neater than making _everything_ an `Arc<HotColdDB>` as I did in #2431).

Supplying the state roots allow us to avoid building a tree hash cache at all when reconstructing historic states, which saves around 1 second flat (regardless of `slots-per-restore-point`). This is a small percentage of worst-case state load times with 200K validators and SPRP=2048 (~15s vs ~16s) but a significant speed-up for more frequent restore points: state loads with SPRP=32 should be now consistently <500ms instead of 1.5s (a ~3x speedup).

## Additional Info

Required by https://github.com/sigp/lighthouse/pull/2628

lcli/src/transition_blocks.rs

@@ -1,7 +1,9 @@
 use clap::ArgMatches;
 use eth2_network_config::Eth2NetworkConfig;
 use ssz::Encode;
-use state_processing::{per_block_processing, per_slot_processing, BlockSignatureStrategy};
+use state_processing::{
+    per_block_processing, per_slot_processing, BlockSignatureStrategy, VerifyBlockRoot,
+};
 use std::fs::File;
 use std::io::prelude::*;
 use std::path::{Path, PathBuf};
@@ -77,6 +79,7 @@ fn do_transition<T: EthSpec>(
         &block,
         None,
         BlockSignatureStrategy::VerifyIndividual,
+        VerifyBlockRoot::True,
         spec,
     )
     .map_err(|e| format!("State transition failed: {:?}", e))?;